There is a formula in Donna Kooler's Encyclopedia of Knitting, but I don't own a copy (I just got mine from the library). It's something like it pulls in 1/6th of a stitch for every so many stitches, or something weird like that. Sorry I can't help more!

All I know is that when working a 5.25" square the cable (8 stitch) will shrink it by 1/4" so maybe its like 1/8" for every 4 stitches (or 1/16 for every stitch, ok so that's exactly what amyb444 said)

I did exactly this - knit a top down turtleneck raglan sweater with a big cable down the front. Here's a picture of it in progress:

I used Merino Chunky -- size 10.5 needles. I did end up starting it, getting a couple repeats down the cable and realizing that the back was growing faster than the front. Then I measured, ripped out and cast on again adding extra stitches in the front to compensate for the pull of the cable. I wrote it down - it's somewhere at home but if you'd like me to search it out for you I'd be happy to. I meant to put the pattern on my blog but never got around to it.

I'm using the same cable panel on both the back and the front, so it's just a matter of making sure I add enough extra stitches so that I don't choke myself.

I've attempted some fancy math, converting inches to percentages, and I think I've figured out how many additional stitches to cast on, using Amethystlily's numbers. But any additional advice would be great!